Stainless steel

ABSTRACT

THIS INVENTION RELATES TO A STAINLESS STEEL AND A METHOD FOR PRODUCING THE SAME, WHEREBY MATERIAL PARTICULARLY ADAPTED FOR USE IN STRUCTRUAL APPLICATIONS SUCH AS THE MANUFACTURE OF CARGO BOXES IS ACHIEVED. SPECIFICALLY, THE MATERIAL IS CHARACTERIZED BY AN IMPROVED COMBINATION OF STRENGTH AND TOUGHNESS THAT IS ACHIEVED BY PRODUCING HOTBAND MATERIAL HAVING A SUBSTANTIALLY MARTENSITC MICROSTRUCTURE OF A COMPOSITION CONSISTING OF 10 MAX. PERCENT CARBON, 2 MAX. PERCENT MANGANESE, 1 MAX. PERCENT NICKEL, 9.5 TO 13.5 PERCENT CHROMIUM, AND THE BALANCE IRON. THIS MATERIAL HAS A MAXIMUM TITANIUM TO CARBON RATIO OF ABOUT 8. WITH TITANIUM TO CARBON RATIOS OF BETWEEN 4 TO 8, NICKEL MUST BE PRESENT WITHIN THE RANGE OF 5 TO 1 PERCENT. FOR OPTIMUM WELD:TOUGHNESS THE MAXIMUM TITANIUM TO CARBON RATIO IS ABOUT 4, EITHER WITH OR WITHOUT NICKEL, TO ACHIEVE THE DESIRED COMBINATION OF STRENGTH AND TOUGHNESS, THE ,ATERIAL IN HOT-BAND GAGE IS ANNEALED FOR A TIME AT TEMPERATURE TO ACHIEVE A HARDNESS OF AT LEAST 80 RB AND PREFERABLY 82 TO 92 RB.

United States Patent 3,832,244 STAINLESS STEEL Kenneth E. Pinnow, J. M. Mehta, and A. Moskowitz, Pittsburgh, Pa., assignors to Crucible Inc., Pittsburgh, Pa.

No Drawing. Original application May 28, 1968, Ser. No. 732,542, now Patent No. 3,778,316. Divided and this application Jan. 11, 1971, Ser. No. 105,690

Int. Cl. C22c 39/14 US. Cl. 148-37 Claims ABSTRACT OF THE DISCLOSURE -This invention relates to a stainless steel and a method for producing the same, whereby material particularly adapted for use in structural applications such as the manufacture of cargo boxes is achieved. Specifically, the material is characterized by an improved combination of strength and toughness that is achieved by producing hotband material having a substantially martensitic microstructure of a composition consisting of .10 max. percent carbon, 2 max. percent manganese, 1 max. percent nickel, 9.5 to 13.5 percent chromium, and the balance iron. This material has a maximum titanium to carbon ratio of about 8. With titanium to carbon ratios of between 4 to 8, nickel must be present within the range of .5 to 1 percent. For optimum weld-toughness the maximum titanium to carbon ratio is about 4, either with or without nickel. To achieve the desired combination of strength and toughness, the material in hot-band gage is annealed for a time at temperature to achieve a hardness of at least 80 R and preferably 82 to 92 R This is a division of Application Ser. No. 732,542,

filed May 28, 1968, and now issued US. Pat. 3,778,316.

For the purpose of satisfying various structural applications, a low-cost stainless steel having a combination of high strength and toughness, while exhibiting satisfactory formability, corrosion resistance and weldability, is desired. Such a material is particularly adapted to the manufacture of cargo boxes for ocean shipping. More specifically, steels for such structural applications are required to be readily formable and weldable without preor postheat treatment. The strength requirements vary with gage but generally such material should have a minimum tensile strength of 65,000 p.s.i. in combination with a minimum yield strength of 45,000 p.s.i., while exhibiting an elongation in 2 inches of a minimum of 20 percent. The material should possess good notch toughness and have at heavy gages a ductile-to-brittle impact transition temperature below 0 F. The material must not only be weldable but must exhibit weld toughness. Weld toughness is essential because structures such as cargo boxes are subjected at the joints to impact loads during service, and improper welding or inadequate weld properties can result in cracking at any notch defects. The corrosion-resistance requirements are not especially stringent; however, the corrosion resistance must be at least sufiicient to make possible the use of low-cost paint systems. Conventional low-alloy 3,832,244 Patented Aug. 27, 1974 45,000 p.s.i., a transition temperature below 0 F. for .25 in. thick material, and a minimum elongation in 2 inches of 20 percent, with good weldability and adequate corrosion resistance.

Still another object of the invention is to provide a stainless steel having a good combination of high strength and toughness, said material being weldable without preor post-heat treatment and said weld heat-affected zone being characterized by good toughness and thus good resistance to cracking upon exposure to impact loads during service.

Yet another object of the invention is to provide a method for producing stainless steel characterized by a combination of high strength and toughness, with good welding properties, by producing as-hot-rolled material having a substantially martensitic microstructure, and thereafter annealing said material for a time at temperature to achieve a hardness of at least R and preferably within the range of 82 to 92 R These and other objects of the invention, as well as a complete understanding thereof, will be apparent from the following description and examples of the invention.

In accordance with the present invention, a steel of the following composition in weight percent is provided:

Carbon .10 max.

Manganese 2max.

Nickel lmax.

Chromium 9.5 to 13.5. Titanium Max. 8 times carbon percent. Iron Balance.

With titanium to carbon ratios of between 4 to 8, nickel should be present within the range of .5 to 1 percent. This is necessary to obtain sufficient martensite in the steelfor the purposes of the invention. The relatively high titanium content within this range combines with carbon to promote ferrite, and thus nickel is required to counter this effect of high titanium.

Within the above-stated broad range, the following preferred ranges may be employed:

Carbon .03 to .08.

Manganese lmax.

Silicon .5 max.

Nickel lmax.

Chromium 10.5 to 12.5.

Titanium Max. 4 times percent carbon. Nitrogen .03 max.

Iron Balance.

Within the above composition limits nickel may be present within the range of .5 to 1 percent. Titanium may be present within the range of .12 to .32 percent. Within these ranges, nickel and titanium may be used singly or in combination.

For the above-listed stainless steels to exhibit the required toughness, it is necessary that they have a substantially martensitic microstructure upon hot rolling to hot-band gage. Thereafter, to produce the required strength, the material in hot-band gage must be annealed at a time at temperature to achieve a hardness of at least about 80 R and preferably 82 to 92 R It has been found, as will be demonstrated by specific examples reported hereinafter, that by annealing to a hardness within this range the above-listed stainless steel compositions will exhibit the required minimum strength. Specifically, minimum tensile strengths of 65,000 p.s.i. in combination with minimum yield strengths on the order of 45,000 p.s.i. with a minimum elongation in 2 inches of 20 percent are produced. As will be demonstrated by the specific examples reported hereinafter, the substantially martensitic structure prior to annealing is necessary to achieve the desired toughness in combination with high strength.

The required substantially martensitic hot-band microstructure is achieved by providing stainless steel within the composition limits, and particularly by adhering to the recited titanium to carbon ratio, after hot rolling the ashot-rolled material is characterized by a substantially To illustrate annealing practices both within and without the scope of the invention and to establish the relationship between the annealed hardness of steel produced in accordance with the present invention and the required strength levels, various stainless steel compositions, within martensitic microstructure, and consequently upon anneal- 5 the composition range of the invention, were melted and ing it exhibits the required toughness. subjected to various anneals as reported in Table III. The To establish the criticality of the martensitic microhardness values for each of these samples is recorded for structure in the practice of the invention in achieving the comparison with the yield and tensile strength of each required toughness, a steel was produced within the comsample. From a comparison of the hardness values with position limits of the invention, except that the titanium the tensile properties for a particular sample, it may be to carbon ratio was 12 to 1 and thus provided a ferritic seen that the desired strength levels are achieved in all hot band. Upon annealing, the steel exhibited an impact instances wherein the hardness values are at least 80 R transition temperature of 50 E, which signifies poor and preferably within the range 82 to 92 R All of the notch toughness. 15 samples exhibited a substantially martensitic microstruc- To specifically demonstrate the present invention, four ture in the hot-band gage, and thus, as reported in Table stainless steel compositions as listed in Table I were III, the toughness of the samples was excellent. However, melted. as may be seen from Table III, to achieve the required TABLE I combination of strength, ductility and toughness, proper Composition percent annealing to achieve a hardness of at least 80 R must be Heat provlded. 0 Mn Si Ni Al N It will be understood, of course, that the specific anneal- 2211: :8; :22 a s2 as :12 :82 a :84; mg t= f in Practice of the m 2217...... .041 .as .32 .25 12.46 .12 .21 .01 1ND .022 partlcularly wlth regard to tune and temperature, W111 2223 .056 11-78 25 depend on various factors, such as the mass and in par- Not detectable when analyzed. ticular the composition of the steel being treated. As may The compositions f Table I were processed in the com be seen from Table III, either strand or box annealing ventional manner from ingot to hot-band gage. Materials may employed 1n the Practlce of the invention- All were hot rolled, at a temperature of about 2100 F., to a that 15 necessary in 811611 Practice s to determine the hot-band gage of about .250 inch. Samples of the materials annealing conditions for a particular material that will in hot-band gage were then subjected to the various anachieve an annealed hardness of at least 80 R If this connealing treatments as listed in Table II. dition is obtained during annealing and if the composition TABLE II Longitudinal tensile-data 0.2% 003- Percent Herdset, yield Tensile elonga- Impact ness, strength strength tionin transition Heat No; Condition Rs (p.s.i.) (p.s.i.) 2in. temperature 2232""; Box-annealed 1,400 F 86 53,600 76,200 26. 0 Below -80 F. 2218 Box-annealed 2217...-.- Strand-annealed 1,500 F-..';.'---.;.- 84 52, 200 80,600 23. 5 Below F.

Box-annealed 1,400 F 77 37, 400 65, 300 29. 0

2223 Box-annealed 1,400 F 86 53,700 80,000 27. 5

It may be seen from the data reported in Table II that, by annealing to achieve hardnesses within the range above recited, the required strength levels are achieved. All of these steels were characterided by a substantially martensitic microstructure in the as-hot-ro1led condition,-

which resulted after annealing in excellent toughness as reported in Table II.

of the steel is within the ranges recited hereinabove to achieve a substantially martensitic microstructure in hotband gage, then the required combination of high strength and toughness will be achieved.

The proper annealing treatment for the purposes of the invention is governed by the composition of the steel, as demonstrated by the data presented in Table IV.

TABLE III Longitudinal tensile-data 0.2% 011- Percent Impact Hardset, yield Tensile elongatransition ness, strength strengt tion in temperature Condition Rb (p.s.i.) (p.s.i. 2 in. F.)

. Box-annealed 1,400 76 41, 300 66, 100 31. 0 Box-annealed 1,475 78 40,400 66, 500 33. 5 Box-annealed 1,400 80 42, 600 9, 500 31. 5 Box-annealed 1,500 80 47,300 72,300 30. 5

Box-annealed 1,475 F 82 54, 300 77, 000 33. 5 50. Strand-annealed 1,500 F--. 84 52, 200 80,600 23. 5 Below 70. Box-annealed 1,400 F...--- 86 53, 600 76,200 26. 0 Below 80. 2227.-.- Strand-annealed 1 600 F.-- 88 59, 200 82, 500 21. 0 Below 60. 2224 Box-annealed 1,400 F 90 65, 85, 300 26. 0 Do. 9918 dn 92 72, 500 87, 900 25. 0

7 than 4 times the carbon content, and the balance iron and incidental impurities.

2. A hot-rolled and annealed stainless steel characterized by an improved combination of strength and toughness and having after hot rolling a substantially martensitic structure and an annealed hardness of at least about 80 R said steel consisting essentially of, in weight percent, .03 to .08 carbon, 1 max. manganese, .5 max. silicon, 10.5 to 12.5 chromium, max. titanium 4 times percent carbon, 1 max. nickel, and the balance substantially iron and incidental impurities in amounts not substantially affecting the properties.

3. The steel of claim 2 having nickel within the range of .5 to 1.

4. The steel of claim 2 having titanium within the range of .12 to .32.

5. The steel of claim 2 having nickel within the range of .5 to 1 and titanium within the range of .12 to .32.

pages 409,

References Cited UNITED STATES PATENTS Aggen 75-128 T Lula 75-126 D Kuehn 75--128 T Phillips 75-126 D Harris 75-128 T OTHER REFERENCES Metals Handbook, Published 1961 by ASM, Vol. I, 

